The present invention relates to a rotation urging mechanism having spring resiliency in the rotating direction and to a pulley device having the rotation urging mechanism.
An alternator which is used as an auxiliary machine of an engine of an automobile or the like is driven by the rotational force fetched from a crankshaft to supply electric power necessary for the traveling of the automobile. A pulley device for an alternator is mounted on an input shaft of the alternator, and the rotational force of the engine is transmitted to the alternator as a belt is trained between the pulley device for an alternator and a pulley mounted on the crankshaft.
In general, the rotational force is imparted to the crankshaft of the engine of the automobile and etc. by the explosive force within a cylinder, variations occur in the rotational speed of the crankshaft. Meanwhile, in the alternator, an armature and etc. which are relatively heavyweight rotate integrally with the input shaft. If the variation of the rotational speed of the crankshaft is rapid, there are cases where the armature is unable to follow the variation of the rotational speed of the crankshaft owing to the inertial force produced by the rotation of itself.
If the alternator is unable to follow the aforementioned variation of the rotational speed, a difference temporarily arises between the rotational speed of the crankshaft and the alternator, in which case slippage can occur between the pulley for an alternator and the belt, or the tension of the belt can vary excessively. Such slippage of the belt and the excessive variation of the tension can cause the generation of abnormal noise of the belt, a decline in its service life, and the like. In addition, there are cases where the initial tension of the belt is sometimes set relatively high to prevent the slippage of the belt, in which case the rotational resistance of the crankshaft increase, possibly lowering the fuel consumption performance of the engine.
For this reason, in order to allow the variation of the rotational speed transmitted from the crankshaft, a conventional pulley device for an alternator is provided with a rotation urging mechanism between a pulley member around which the belt is wound and a pulley boss which is integrally fixed to an input shaft of the alternator rotatably.
As the rotation urging mechanism, JP-A-5-180287 discloses a technique in which a torsion coil spring is provided between the pulley member and the pulley boss, and the pulley member and the pulley boss are temporarily relatively rotated resiliently in the circumferential direction to thereby alleviate the variation of the rotational speed.
With the rotation urging mechanism of JP-A-5-180287, the alleviation characteristic of the variation of its rotation with respect to the pulley boss of the pulley member is dependent on the torsion coil spring. However, since this torsion coil spring is limited to the size of such a measure that the torsion coil spring can be built into that pulley device for an alternator, its wire diameter, free length, number of turns, and the like are limited, so that the characteristics of the torsion coil spring cannot be set freely. In addition, the spring constant of the torsion coil spring is fixed, and therefore the spring constant cannot be varied freely in accordance with the helix angle. For this reason, the degree of freedom in setting the alleviation characteristic of the variation of rotation as the pulley device for an alternator is unfavorably limited. Hence, there is a possibility that the variation of the rotational speed of the crankshaft cannot be alleviated sufficiently. In addition, owing to the provision of the coil spring, the strength of the pulley device declines, which is disadvantageous in terms of the durability.